Electric vehicles, including battery, hybrid, and fuel cell electric vehicles, typically use an inverter in the form of a switch-mode power supply to provide three phase operating power to the vehicle's electric drive motor. The inverter design most commonly used is a pulse width modulated (PWM) voltage source inverter which utilizes power transistors that can supply the high currents needed to satisfy the torque demands required by the vehicle drive motor. The inverter switches power to the motor windings from a direct current (dc) bus. For a low voltage system, the dc bus is typically 42V, while for a high voltage system the dc bus is approximately 350-400 volts (Vdc).
When different phases of the inverter switch, they cause significant fluctuations in the power supply voltage. Collectively, these fluctuations are known as power bus voltage and current ripples. Typically large capacitors or capacitor banks connected in parallel to the direct current (DC) power supply are used to compensate for power bus ripples by functioning as a smoothing filter. The size of the capacitor bank in an automotive inverter system depends on the bus voltage, power level, and the ripple tolerance of the system. In the automotive environment when driving an inverter from a high voltage bus, a capacitor of 1000 micro Farads (μF) or larger may be required to smooth the voltage ripple sufficiently. The amount of capacitance can be in excess of 16,000 μF for a low voltage system, such as a 42V system. A capacitor or capacitor bank having a value this high requires a significant amount of space and increases vehicle cost.
Power bus ripples are higher for multiple inverter systems. Multiple inverter systems are systems with two or more inverters that drive a corresponding number of motors but are powered from the same energy source. For example, in hybrid vehicles dual inverter systems may be used for the transmission system or for the traction system. The power bus ripple problem is compounded in multiple inverter systems because the ripple is additive. Thus, in order to power both inverter systems from the same power supply, it is necessary to provide a larger capacitor than would be required for a single inverter system alone. It would be desirable to have a multiple inverter system that requires a smaller capacitor to provide a given degree of smoothing. Such an inverter system is provided by the present invention, whose features and advantages will be more clearly understood from the following detailed description taken in conjunction with accompanying drawings.